SINKER EDM BASIC TERMINOLOGY – PART II – FLUSHING
MAR 2018 Blog

It’s been said the 3 most critical things about a good sinker EDM burn condition are flushing, flushing, and flushing. This is still true today but modern machines have built in technologies that assist with flushing even when the operator doesn’t think about flushing. More on these in a bit.

Flushing is a critical part of the EDM process, as it removes contaminated fluid and eroded particles, and replaces them with clean, temperature controlled fluid. This removes contamination from the spark gap that could develop into undesirable conditions like slow, unstable burns, pitting of the work piece, and possibly destructive DC arcing. Flushing also helps cool the work piece by replacing warmed EDM fluid with chilled fluid. Spark temperatures, even though extremely small, can be in the 15,0000 to 22,0000 F range.

There are a few basic types of flushing. The first discussed here will be Injection Flushing, called such because cooled dielectric fluid is injected through the electrode into the spark gap. This is the most efficient type of flushing for many jobs as it basically pressurizes the spark gap around the electrode to keep a continuous flow of fluid going from the bottom of the cavity up the sides, and out the top. This removes eroded particles and keeps the work area cool. Injection flushing is particularly effective for large, deep, and/or complex shaped burns.

Things to be careful of when using injection flushing:

• Over burn at the top of the cavity due to secondary erosion. This is caused by excess conductive debris being washed out of the burn making the sparks longer.
• A post created where the flushing hole is. This can become unstable as it gets taller and moves around under flushing pressure inside the flushing hole. This can cause sporadic short circuit problems with the burn.
• The area around the flushing holes can become distorted and worn unevenly due to the flow and pressure of the flushing.

Another basic flushing style is Suction Flushing. With suction flushing, a vacuum line is attached to the bottom of a cavity or detail being burned, and the fluid and debris flows downward to the suction point. Suction flushing is very effective in burns with materials that develop smaller debris particles, like carbides. Many times suction flushing will be used together with a part holding fixture, and it is not uncommon to see suction and injection flushing used together on some applications.The last basic flushing style covered here will be Lateral Flushing. Also referred to as Cross Flushing or Sweep Flushing, this form of flushing uses external nozzles to cause of flow of dielectric fluid across the part being EDM’d. Lateral flushing is, in general, the least effective form of flushing discussed here. IF the burn is very shallow and not too big of an area, lateral flushing is effective. It is also effective in setting up flow directions with some burns. Flushing nozzles placed in opposite corners of a cavity can cause a swirl effect around the entire electrode. As with injection flushing, a drawback is deformation of the electrode by the flow and impact of the fluid eroding the electrode in some areas.

Back to those built in technologies I mentioned earlier. Many modern EDM machines have several technologies that assist with or compensate for poor flushing situations.

• High speed axis drives
Can accelerate/decelerate in multiple-G range
Causes a stirring of fluid and debris in the spark gap
Fresh cool fluid sucked in upon retraction as dirty warmed fluid is forced out

• Rotating C-Axis spindles
Extremely effective in drilling deep holes
Rotation of electrode causes swirling of fluid, keeping debris from gathering in one place

• Highly adaptive advanced control systems
Monitors spark activity in real time
Makes changes to spark parameters to prevent a bad condition in the spark gap

• Programmable flushing systems built into machines

No matter how its achieved, flushing is still a critical part of the EDM process. For more information refer to your machine’s user manual or technical support.